1. Field of the Invention
This patent specification relates generally to a system and a method for data communication, and more specifically to such system and method incorporating a host computer and a plurality of terminals interconnected to each other by way of telephone lines including the public switched telephone network.
2. Discussion of the Invention
Data communication systems including a plurality of terminal units and a host computer are well known. The terminal units are remotely located and provided with communication capability, and are connected to the host computer by way of telecommunication networks, including public switched telephone networks.
When a request for a line connection is issued in the communication system from the remotely located terminal to the host computer, the validity of the line connection is determined by the host computer before initiating the data communication, in which the confirmation is generally carried out after establishing the connection via the line connection.
With this system structure, which includes a plurality of terminals and a host computer, a line connection is established between these terminals and the host computer only over the period when necessary, during which data communication is then carried out.
All of these line connections, however, are not always valid, since the terminals requesting line connections are not all necessarily uniform. As a result, there is a difficulty in the system, in that requests for line connections have to be met even for terminals for which the connection is not allowed (or invalid).
In addition, there is another difficulty in the system, in that some of the terminals are outside management parameters for the system, caused, for example, by a change in the working environment of the system on the part of the customer. This is exemplified by a change in phone number, alteration of the mode of telephone line (rotary dialing or pushbutton dialing), and incorporation of a private branch exchange (PBX), which are carried out in general independent of setting conditions of the data communication system.
Therefore, if a current setting is made such that data communication is performed using an already existing facsimile (FAX) line and only over the period other than when used for the FAX transmission, access for the data communication system may not be achieved. That is, the host computer is not able to reach the desired terminal to communicate, and this may be encountered when a change in the aforementioned environmental conditions for the system is made such as, for example, in the corporate organization (or system), in the configuration of the data communication system, in the wiring of communication lines, etc.
In such a case, the data communication system suffers from a drawback, in which there is no other way for the host computer to receive a call from that terminal. It becomes therefore unfeasible for the system to determine the reason why the terminal is non-accessible, in cases where the terminal is not presently operational, or is disconnected due to line displacement or removal, for example.